At the moment it is about 10000 tris (and will be about that in the end) but if it is really needed to be 6k I can remove some geometry. I have some questions on this subject:

I would like to know if it is needed to model inside of the cabin or should I just cover all the windows with plating as in the concept art? At the moment it is possible to open the doors, though the cabin is empty. Removing this capability would reduce the polycount quite a lot as the doors could be just in the textures.

Also, how much of the underside should be modeled? Are vehicles just immobile meshes or can they move/explode/flip etc?
If the vehicle should be able to move around, should I build the rig for animating?

(image above has the "no doors cabin" + one of the doors as floating geometry just to get an idea how the normals will look)